mmc: block: remove the check of packed for packed request routine

packed should always exist without calling its cleanup function
explicitly. Moreover, we have use it when preparing packed list.
So I don't believe we should ever fall into this check again when
doing mmc_blk_packed_hdr_wrq_prep or mmc_blk_end_packed_req,etc.
And the code of mmc_blk_end_packed_req is trying to use packed before
checking it which makes it quite weird. This patch is trying to
remove these two checks and move it to the mmc_blk_prep_packed_list.
If we find packed is null, then we should never use MMC_BLK_PACKED_CMD.
By doing this, we could fall back to non-packed request if finding null
packed, though it's impossible theoretically.

After removing these two BUG_ONs, we also remove all other similar
checks within the routine of mmc_blk_issue_rw_rq which checks the
error handling of packed request.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>